Aerosol particles affect the Earth’s radiative balance by directly scattering and absorbing solar radiation and, indirectly, through their activation into cloud droplets. Both effects are known with considerable uncertainty only, and translate into even bigger uncertainties in future climate predictions. More than a decade ago, variations in galactic cosmic rays were suggested to closely correlate with variations in atmospheric cloud cover and therefore constitute a driving force behind aerosol-cloud-climate interactions. Later, the enhancement of atmospheric aerosol particle formation by ions generated from cosmic rays was proposed as a physical mechanism explaining this correlation. Here, we report unique observations on atmospheric aerosol formation based on measurements at the SMEAR II station, Finland, over a solar cycle (years 1996–2008) that shed new light on these presumed relationships. Our analysis shows that none of the quantities related to aerosol formation correlates with the cosmic ray-induced ionisation intensity (CRII). We also examined the contribution of ions to new particle formation on the basis of novel ground-based and airborne observations. A consistent result is that ion-induced formation contributes typically less than 10% to the number of new particles, which would explain the missing correlation between CRII and aerosol formation. Our main conclusion is that galactic cosmic rays appear to play a minor role for atmospheric aerosol formation, and so for the connected aerosol-climate effects as well.

And their conclusion:

Galactic cosmic rays and the related ion-induced nucleation have been proposed to be among the key factors governing atmospheric aerosol budgets and subsequently cloudiness and global climate. Here we have shown, based on long-term experimental data, that atmospheric nucleation frequency or nucleation mode particle concentrations do not show correlation with galactic cosmic rays on either yearly or monthly basis. The geomagnetic activity showed similar seasonal behaviour as nucleation event frequencies, peaking in spring and autumn, but this similarity seems to be caused by different reasons. Accordingly, no significant daily correlation between these variables was found. Our results do not support the idea that the ions produced by galactic cosmic rays would be a major factor behind secondary aerosol production and the related aerosol-cloud interactions.